1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which feedback-controls the air-fuel ratio of a mixture supplied to the engine cylinder by cylinder, by means of feedback control to which an observer based on a modern control theory is applied.
2. Prior Art
Conventionally, there has been proposed an air-fuel ratio control system for internal combustion engines, for example, by Japanese Laid-Open Patent (Kokai) No. 7-259588, which applies an observer which estimates the air-fuel ratio of a mixture supplied to each cylinder (cylinder-by-cylinder air-fuel ratio), based on an output from air-fuel ratio-detecting means, arranged in the exhaust system of the engine at a confluent portion thereof, for generating an output proportional to the air-fuel ratio of exhaust gases, and calculates an air-fuel ratio control amount based on the estimated air-fuel ratio values so as to eliminate variations in the controlled air-fuel ratio between the cylinders. According to the proposed air-fuel ratio control system, values of the output from the air-fuel ratio-detecting means are sequentially sampled and stored, and the sampling timing of the output from the air-fuel ratio-detecting means is corrected based on operating conditions of the engine and a degree of deterioration of the air-fuel ratio-detecting means, to select a value of the output from the stored values according to the corrected sampling timing, to thereby always detect the optimum air-fuel ratio. In this proposed air-fuel ratio control system, when fuel-cut (interruption of fuel supply to the engine) is carried out, a delay time from the actual start of the fuel-cut to the time the output from the air-fuel ratio-detecting means indicates a lean air-fuel ratio value is actually detected, and a change in the response speed of the air-fuel ratio-detecting means is detected based on the detected delay time.
According to this manner of detecting the response speed change, however, values of the output from the air-fuel ratio-detecting means must be monitored at very short sampling intervals in order to accurately detect the delay time, which requires a specific detecting process executed at time intervals (repetition period) much shorter than those of an ordinary air-fuel ratio control process.
Further, when the response speed change of the air-fuel ratio-detecting means exceeds an expected degree, correction of the sampling timing cannot be executed, which can result in inaccurate estimation of the cylinder-by-cylinder air-fuel ratio. In such a case, if the feedback control is executed based on the estimated cylinder-by-cylinder air-fuel ratio, it can result in increased variations in the controlled air-fuel ratio between the cylinders.